Nowadays, scraper-chain conveyors and chain driven machinery for mineral, e.g. coal, mining installations require powerful drive systems equipped with overload protection mechanisms usually hydraulically actuated multi-disc clutches. Overload protective gearing is known and has been tried and test in mining practice. Such known gearing has an integrated multi-disc friction clutch driven by compressed air via an electro-hydraulic quick-acting valve which operates in the event of overload. Overloading is detected with measured value transmitters which either measure the torque on the gearing output shaft or measure the speed differences in the motor or in the gearing by means of proximity switches. The prevailing chain drive force is thus continuously monitored during operation.
Known overload protective gearing consists either of two-stage spur gearing, two-stage planetary gearing or of gearing having one planetary stage and one spur wheel gear stage. In gearing with a planetary stage, the safety clutch can be allocated to one of the normally operating planetary gear components usually the hollow toothed wheel or ring gear on of which a supporting moment is adjusted by the clutch in relation to the stationary housing.
Examples of known gearing and equipment for drive systems with overload protection are described in U.S. Pat. Nos. 4682678, 4614134 and 4667790 and in DE 3722612.
It is known also in chain drive systems to have separate motors and overload protective gearing forming main and auxiliary drive stations or units and to use the overload protective gearing to compensate for load variations in order to adapt the motor speeds and therefore the motor ratings on the main and auxiliary drive units, see for example, DE-3733336, DE-37140762 and U.S. Pat. Nos. 4939949 and 5055092. One known load compensating gearing uses a supportive motor which is allocated to the ring gear or the planet carrier of a planetary gearing stage. It is also already known in the case of load compensating gearing to allocate to the safety clutch controller a slip monitoring device and a speed monitoring device of which the measured values are evaluated by the controller for switching the safety clutch in the case of chain blocking.
In all known drive systems for powerful haulage chains or conveyor drives considerable load peaks accompanied by significant stresses arise when starting under load. Hydraulic couplings can be used as a starting aid in order to achieve smooth starting of the conveyor or the haulage chain but the use of hydraulic couplings has not been a practical solution.